1. Field of the Invention
The present invention relates generally to an apparatus for reducing the amount of electric power consumed by large electric motors during startup and, more particularly, to a fluid power auxiliary system for starting such motors.
2. Description of the Prior Art
It is well known that electric motors and, in particular large electric motors connected to relatively heavy prime mover loads such as commercial air conditioning or refrigeration compressors, ventilating fans and the like consume a great deal more electric power when starting and coming up to speed than they do when running at a steady state condition. In addition to the initial power surge or "spike" associated with the initial starting of such motors, which may cause an inrush of power equal to two or three times the power consumed when such a motor is running at constant speed, these motors consume power very inefficiently from the time they are switched on until they are brought up to operating speed. The motors themselves, of course, also must be built with heavy starting windings to accommodate the extra power required for starting.
Many commercial installations have a number of such motors located in close proximity. The wiring network supplying the motors, of course, must be sized to accommodate the initial power surge or spike or other highest rate of power consumption so as to prevent any overloading of the system when the motors are started. These characteristics lead to high initial costs occasioned by the need for heavier wiring in both the supply network and the motors themselves.
Typically, such motors are controlled by temperature sensors or the like and are prone to cycle on and off quite frequently. This increases the percent of operating time which is spent in starting and bringing the motors up to operating speed. Because of the surges and inefficiencies associated with startup, the overall power consumption of the system may be increased significantly. In recent times, the increasing costs of power generally coupled with the increasing requirements for load-shedding during peak power consumption intervals has greatly increased the need for reducing or eliminating power consumption inefficiencies. If the start up surges and operating inefficiencies can be reduced or eliminated a great deal of cost savings can be realized in both the wiring networks and the motors themselves by eliminating the need for allowing for the higher power consumption in the network and the oversized motor windings. In addition, the total average amount of power consumed by the large user can be greatly reduced.
In addition to the recognized increase in power consumption, excessive starting and stopping of electric motors associated with short on-off cycles tends to shorten the life of such motors. This is due to excessive heat build-up in the windings and bearings. In fact, because of this problem, the allowable time between motor starts has to be lengthened beyond that which economically fits the application, in certain cases, to allow the motors to cool properly before being restarted. If such motors could be started by auxiliary means, winding and bearing life could be increased and the necessity for relatively long cooldown periods could be avoided.
Thus, there exists a definite need for an auxiliary motor starting system which operates on a reduced amount of electric power or utilizes stored power which can be released to start such motors and bring them at least partially up to operating speed before the power is turned on to the motors themselves. One prior art system is illustrated and described in U.S. Pat. No. 3,300,697 in which a hydraulic cylinder is used to operate a pawl against a ratchet wheel to start the rotation of a lathe or the like used for machining very heavy workpieces such as steel mill rolls. That system, however, aids in overcoming only the inertia of the stopped spindle where even the full available torque is insufficient to start rotation.